FIG. 1 depicts a schematic diagram of an exemplary wireless local-area network (LAN) 100 in the prior art comprising access point 101 and stations 102-1 through 102-N, wherein N is a positive integer, interconnected as shown. Each station 102-i, wherein i is a member of the set {1, 2, . . . N}, is a device such as a notebook computer, personal digital assistant (PDA), tablet PC, etc. that transmits radio signals to and receives radio signals from other stations in local-area network 100 via access point 101.
Access point 101 and stations 102-1 through 102-N transmit data in units referred to as frames over a shared-communications channel such that if two or more stations (or an access point and a station) transmit frames simultaneously, then one or more of the frames can become corrupted (resulting in a collision). As a result, local-area networks typically employ one or more protocols to ensure that a station or access point can gain exclusive access to the shared-communications channel for an interval of time in order to transmit its frames. Frames transmitted from a station 102-i to access point 101 are referred to as uplink frames, and frames transmitted from access point 101 to a station 102-i are referred to as downlink frames. In accordance with some protocols (e.g., Institute of Electrical and Electronics Engineers [IEEE] 802.11, etc.), when a station 102-i receives a downlink frame from access point 101, the station transmits an acknowledgement back to access point 101 informing the access point that the downlink frame was successfully received. Similarly, when access point 101 receives an uplink frame from a station 102-i, the access point acknowledges receipt of the frame. Acknowledgements may be combined with data frames, the receipt of which must be acknowledged, too.
A station 102-i can prolong its battery life by powering off its radio (or in general, its transceiver) when not transmitting or receiving. When a station powers off its radio, the station is said to enter a reduced-power state (also called the doze state). A station wakes up from the doze state by powering on its radio to enter the alert state. While a station is in the doze state, it cannot transmit or receive signals, and is said to be asleep. While a station is in the alert state, it can transmit or receive signals; the time interval that a station is known to the access point to be in the alert state is said to be the awake period. A station that conserves battery life by alternating between alert and doze states is said to be in power-save mode, and a station that employs power-save mode is said to be a power-saving station.
Problems can arise, however, when power-saving stations enter the doze state. Typically, the protocol enables the station to know whether additional frames will be transmitted to the power-saving station in the alert state before it goes back to sleep. There are several ways for a power saving station to know that a frame is the last frame to be sent by the access point before it wakes up again. For example, according to the IEEE 802.11-1999 protocol, a power-saving station, which sends a frame known as the PS-Poll to notify the access point that it is awake, will receive a single downlink frame from the access point. In another example, a power-saving station that follows the IEEE 802.11e proposed power-saving mechanisms will be notified that a downlink frame is the last frame received in its awake period by the access point setting a special bit in the control field of that frame to 1.
Once a station has received the last frames destined for it, it may enter the reduced-power state. However, a problem can arise when a power-saving station, after receiving the last downlink frame, transmits an acknowledgement to the access point and subsequently enters the doze state. In particular, in accordance with some protocols, if the acknowledgement is “lost” (i.e., the access point doesn't receive the acknowledgement [e.g., due to RF interference, etc.]), then the access point re-transmits the frame to the power-saving station and waits again for an acknowledgement. Because the power-saving station is in the doze state, however, the station does not receive the re-transmitted frame, and therefore does not transmit an acknowledgement to the access point, thereby resulting again in a lost acknowledgement, which causes the access point to re-transmit the frame, etc. The retransmissions repeat indefinitely or until a pre-specified limit on retransmissions is exhausted. The occurrence of such “repeated” lost acknowledgements causes, at the very least, a waste of bandwidth, and potentially, depending on the protocol, a significant increase in delay, jitter, etc. for other stations in the local-area network.